1. Field of the Invention
The present invention relates to a laser surgery system. More particularly, it relates to a non-contact laser ablation method and apparatus providing laser fluence compensation of a curved surface, especially a corneal surface.
2. Background
The cornea and lens of an eye act in unison on light entering the eye to focus the incoming light onto the retina. When the refractive power of the cornea and lens are optimized for the length of the eye, a sharp image is focused on the retina. Myopia (nearsightedness) is the result of blurred images caused when the focal point of the image is located before the retina. Hyperopia (farsightedness) is the result of blurred images caused when the focal point of the image is behind the retina. Astigmatism is a unique refractive error that causes reduced visual acuity and produces symptoms such as glare, monocular diplopia, asthenopia and distortion and occurs when the focus from tangential light rays are at a different point than the focus of the sagital light rays.
Vision acuities result from refractive errors from the corneal of the eye and the lens within the globe of the eye. For example, nearsightedness, or myopia is a result of the shape of the corneal membrane being too steep.
One popular technique for correcting vision acuities is reshaping the cornea of the eye. The cornea is chosen for modification before other components of the eye because it is the strongest refracting component of the eye and is accessible without interoccular surgery. As an example, the cornea of a patient with hyperopia, or farsightedness is relatively flat resulting in a large spherical radius of the cornea. A flat cornea creates an optical system that does not correctly focus the viewed image onto the retina of the eye but in fact the focal point is beyond the surface of the retina. Hyperopia can be corrected by reshaping the eye to decrease the spherical radius of the cornea. In the case of correcting hyperopia, corneal tissue is typically not removed at the center of the cornea but is removed deeply at the periphery of the cornea.
As another example, to correct myopic effects of an eye, procedures are performed which effectively increase the radius of the cornea. In this case, the corneal surface is removed deeply at its center and slightly at its periphery.
In another example, such as the case of the correction of astigmatism (e.g., myopic astigmatism), the surface of the cornea is removed deeply at its center but only along a certain axis and slightly at its periphery. The resulting shape of the cornea is that of a cylindrical convex lens.
Changing ablation patterns on the cornea performs the various vision corrections. Use of an ablating laser beam for removing the surface of the cornea to correct ametropia of any sort requires precise administration of the laser beam. Optical systems are commonly used to control or condition an ablating laser beam exiting from a laser source prior to impingement onto a corneal surface.
A common ablation laser system scans and pulsates an ablating laser beam across a corneal surface. Typically, the laser source is fixed in location with respect to the patient""s eye, as is the patient. To remove corneal tissue throughout a given ablation pattern, the ablating laser beam is typically directed across the corneal surface with the use of scanning mirrors.
However, as is appreciated by the present invention, ablation of a curved surface introduces several dynamics that are typically unaccounted for in conventional laser ablation systems.
For instance, as shown in FIG. 4A, when the ablating laser beam is ablating a spot on the cornea directly below, a xe2x80x98directxe2x80x99 hit on the cornea causes a maximum amount of energy absorption and transfer between the laser beam and the corneal tissue being treated. This is because a normal or perpendicular angle xcex8 is formed between the laser beam and the surgical plane. However, as the angle of the laser beam with respect to the surgical plane changes from 90xc2x0 as shown in FIG. 4B, less energy from the laser beam transfers to the corneal tissue, resulting in changing depths of ablation across the ablated curved surface.
Generally speaking, the eye is a spherical surface as depicted in FIGS. 4A and 4B, and the angle of incidence of the scanning laser beam on the eye varies as the with respect to distance from the apex of the eye. FIGS. 4A and 4B illustrate that the farther the apex of the laser beam is from the center of the targeted curved surface 10, the greater the angle of incidence 2xcex8xe2x80x2 of the laser beam due to the curved surface 10. This is especially true for small beam, scanning laser ablation systems, although it is also true for broad beam systems. The broad beam systems must compensate for the loss of power transferred to the cornea at the periphery.
FIG. 4B is illustrative of the reflection of additional laser energy off the curved irradiation site due to the enlarged spot size being projected onto the cornea.
As appreciated by the present inventor, as an ablating laser beam spot traverses a corneal surface, it tends to become elliptical on the curved surface, and assumes a larger area. Fluence is defined as energy over area, or energy density. Thus, as the laser beam angles steeper and steeper (i.e., further from a normal to the surgical plane) at the edges of a larger and larger ablation pattern, the fluence decreases. This is appreciated to result in ablation depths toward the edges of the ablation pattern which are less than the expected depth, and less than the ablation depth at a central portion of the ablation pattern at a point directly below a normal angled laser beam.
The increased depth per pulse in the central portion of the ablation pattern as opposed to the peripheral portions of the ablation pattern often cause the resulting shape to be non spherical and will change the prolate nature of the cornea. The non-uniform removal of tissue (e.g., corneal tissue) can produce an irregular corneal outer surface and may even prevent proper healing.
There is a need for an ablation apparatus and method, which provides greater control and uniformity of the depth of ablation across an ablation pattern on a curved surface such as a corneal surface.
In accordance with the principles of the present invention, an ablation laser system having variable fluence comprises a laser source, and relay optics for delivering a laser beam from the laser source to a target surface. The number of pulses is increased in the periphery to compensate for the reduced ablation due to reduced fluence in this region.
In accordance with another aspect of the present invention, an ablation laser system having variable fluence comprises a laser source, and relay optics for delivering a laser beam from the laser source to a target surface. An ablation spot fluence adjuster adjusts a fluence of an ablation pulse on the target surface.
In accordance with another aspect of the present invention, a system for imparting ablating laser radiation onto a target curved surface comprises a laser source having an output laser beam, and a variable aperture device. A controller is operatively connected to the aperture to adjust the diameter of the laser beam. Relay optics produce an image of the laser beam, and turning optics scan the image of the laser beam across the target surface.
A method for providing laser radiation on a curved surface having a desired fluence throughout in accordance with yet another aspect of the present invention comprises providing an ablating laser beam. A cross-sectional shape of the ablating laser beam is set to a first size, with respect to a particular ablation spot of a particular ablation pattern on a particular layer of tissue to provide a given fluence level for that particular ablation spot. The ablating laser beam is scanned to another ablation spot of the particular ablation pattern on the particular layer of tissue, and the cross-sectional shape of the ablating laser beam is re-adjusted to a second size different from the first size, with respect to another ablation spot, to maintain the given fluence level for the ablation spot.